Too much fuel pressure, leaky injectors. That's all I can think of. Running your engine like that will DESTROY the catalytic converter. It will overheat and collapse pretty quickly. Very unusual for a FI engine to dump gas like that. An exhaust leak shouldn't matter.
98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54